In recent years, gas-liquid mixture fluids containing micro bubbles have been used in various industries, for example: to dissolve a dioxin water mass in a closed water area, as an activation means of microorganisms in drainage treatment, for facilitation of the growth of plants in hydroponics and the like, for removal of contaminating substances on the surfaces of a material, and as a technique capable of supplying various gases into water by making such gases in the form of micro bubbles.
Air oxidation in a liquid phase is one of the reactions most commonly employed in various industrial processes. In carrying out this reaction, air is generally blown under pressure into water through fine pores of a tubular or planar micro-bubble generating system installed at the bottom or in the lower portion of the side wall of a tank.
A known design of bubble generator comprises a bubble plate which is provided with an air chamber to be connected to a source of compressed gas. The air chamber has a bubble generating face comprising a number of openings. When the bubble generator is immersed in liquid air in the chamber, which escapes through the holes forms bubbles in the liquid. A mechanical arrangement is provided to change the size of the openings in the bubble generating face and therefore the diameter of the bubbles produced. Bubble generators having this design are described in EP0029814B1, and U.S. Pat. No. 4,269,797; U.S. Pat. No. 5,110,512.
It must be noted that in aeration systems using this conventional type of bubble generating system, even when fine pores are provided in the bubble plate, the volume of each of the air bubbles expands and the diameter of each bubble increases to several millimeters due to surface tension of the air bubbles during injection. When bubbles are supplied into water, it is known that for the same volume of gas, minimizing the outer diameter of the bubbles increases the surface area of the bubbles relative to their, thereby enlarging the contact area between the gas in the bubbles and the water.
The disadvantages of this type of bubble generator are difficulty of forming bubbles with small diameter, clogging of the small pores, and the high energy consumption required to generate the bubbles. This type of bubble forming means also fails to provide enough rapid and efficient oxidation.
The introduction of gas/liquid two-phase flow has been shown to significantly enhance the performance of some membrane process applications (Cui Z. F., Chang S., Fane A. G., “The use of gas bubbling to enhance membrane process”. J. of Memb. Sci., 221, 2003). The basic mechanism of the enhancement by gas sparging was attributed to the secondary flow induced by air bubbles (Li Q. Y., Cut Z. F., Pepper D. S., “Effect of bubble size and frequency on permeate flux in gas sparged ultrafiltration with tubular membranes”. Chem. Eng. J. 67, 1997). The secondary flow around the air bubbles then promotes local mixing, reduces the thickness of the mass transfer boundary layer and increases the mass transfer coefficient. Consequently, the mass transfer rate of solute molecules from the membrane surface back to the bulk solution is increased. Injecting air bubbles can increase the permeate fluxes by 7% to 50% and higher (Ghosh R., etc., “Enhancement of ultrafiltration by gas sparging with flat sheet membrane modules”. J. Separation and Purification Technology 14, 1998). The complicated designs of these existing systems and increased consumption of energy by them are disadvantages of this technology.
U.S. Pat. No. 6,382,601 describe a bubble generator design, which overcomes these disadvantages. The bubble generator comprises a tangential channel through which water is supplied to a conical space through a cylindrical inner wall with sufficient volume and pressure to develop a vortex in the flowing liquid and gas is introduced into the flowing liquid orthogonally through a porous wall. U.S. Pat. No. 4,618,350 propose creation of two phase flow by injection in the vessel gas at high tangential velocity. The large centrifugal force field ensures that the relatively dense liquid droplets spiral outwards, counter-current to the inwardly-spiraling gas.
The most vital portion of the generator related to the bubble generation in the swirling jet method is the flow region in the vicinity of the jet exit. A swirling type micro-bubble generating system described in U.S. Pat. No. 7,832,028 is constructed by arranging two fine-bubble generating sections in a rectangular parallelepiped casing with spouts of fine bubble generating sections facing each other. The bubble generating sections have chambers containing rotating liquid with gas introducing passages opened into each of the chambers through partition walls separating them. According to the inventors of this system, it is possible to readily generate micro-bubbles in industrial scale, and the system is relatively small in size and has simple structure and can be easily manufactured.
Disadvantages of the bubble generator of U.S. Pat. No. 7,832,028 are the air is introduced into vortex chamber on the vortex chamber axis, which doesn't permit using water having high tangential velocity for creating bubbles. Also, the long vortex chamber reduces the rotational velocity in the liquid as a result of angular momentum losses. This reduces the efficiency of the bubble generator design and increases the energy consumption. In this design is difficult to optimize the geometrical parameters and a great deal of experimental work must be done to obtain the requested results.
One of the most effective approaches for providing good liquid filtering along with creating two phase flow is to induce liquid instability near the membrane surface by using an intermittent jet or pulsating flow. Among the mechanisms proposed to explain the enhanced performance is one that suggests that pulsation produced enhanced shear when flow reversal of the bulk solution was achieved (Ding L. H., Jaffrin M. Y., and Defossez M., “Concentration polarization formation in ultrafiltration of blood and plasma”, J. Memb. Sci, 84, 293, 1993). It is postulated by Ding et al. that, after a stable pressure period during which a cake was formed, a low-pressure peak caused destabilization of the cake layer. Then during the high-pressure peak, high shear removed many particles from the cake layer. It has been demonstrated experimentally that with a waveform in which the high pressure peak followed the stable pressure period, the high maximum shear removed fewer particles from the cake layer than when the boundary layer was destabilized first (Gupta B. B., Blanpain P., Jaffrin M. Y., “Permeate flux enhancement by pressure and flow pulsations in microfiltration with mineral membranes”. J. Memb. Sci., 70, 256, 1992). When an intermittent jet is used, the generation and characteristics of the vortices essentially arise from the inertial effects due to the difference in velocity between the jet and the surrounding liquid. The shear stresses coming from viscosity effects or turbulence will affect the development of these vortices and the rapidity of transfer of the energy they contain result in smaller and smaller eddies.
One of the methods for creating pulsating flow for improving membrane separation process and an apparatus for its realization is proposed in U.S. Pat. No. 6,613,231. The method comprises using four flat fixed membranes, at least three bodies rotating next to the selective layer of the membranes that generate secondary vortices, and providing at least one device that generates oscillations in the liquid. Flow rate in the intermediate cell will increase or decrease cyclically as the impeller of the device set on a solid or hollow shaft rotates around its axis, and allows obtain oscillating liquid flow in each intermediate cell of the apparatus for membrane separation. The aforementioned oscillatory conditions in liquid facilitate removal of liquid from membrane selective layers, thus enabling to ensure high specific permeability for a long period of time. This means that it is possible to reduce the frequency of washing and replacement of membranes.
U.S. Pat. No. 6,962,169 and U.S. Pat. No. 7,887,702 describe apparatuses, in which pulsate flow is created by a rotation element through which flow is supplied to membrane. The device comprises a liquid inlet, a liquid outlet and a blocking element that is located between the liquid inlet and the liquid outlet and that rotates about an axis. The blocking element comprises a blocking member which cyclically closes and opens a liquid passage from the inlet to the outlet. In the device the pulsating frequency of the resultant pulsating liquid stream corresponds to twice the rotational frequency of the blocking element. Apparatuses which use this method are mechanically complicated and experience problems of wear, bulkiness' and adjustment—particularly concerning the rotating parts.
U.S. Pat. No. 4,512,514 describe a method for creating a pulsating liquid flow conditions by using an elastic element overlapping the supply channels. One of the embodiments of a device that implements this method consists of an elastic tube and a casing. The casing surrounds the elastic tube and forms a space between the inner surface of the rigid casing and the outer surface of the elastic tube. Liquid flows into the pulsator through its inlet at a low controlled continuous flow rate and is ejected through its outlet at a high intermittent pulsating flow. In this arrangement the volume of air surrounding the elastic tube and enclosed in the casing is compressed during the expansion of the elastic tube. A significant drawback of the proposal is the need to supply to a liquid or gas at a pressure that is greater than the pressure in the flow which must be to filtering. In addition, the introduction of the device of elastic deformable element does not allow the creation of flow pulsation with high frequency. This device is also limited in the length of time it can operate.
According to its design, the easiest way to create a pulsating flow is proposed in US 2006/0102234. This patent application describes a flow control device that creates a pulsating flow. The device includes a body with a flow passage defined there through, a flow interruption element and means, for example, a spring that moves the flow interruption element between a flow interrupting position and an open position. Under the influence of differential pressure acting on the element, the spring is compressed opening passage for liquid. After a rise in pressure in the output cavity the force of the spring on the element will push the element to block passage of the liquid. Despite this simple construction this method for creating pulsed liquid flow regime has several disadvantages, which primarily relate to time constraints on the action of the device, limitation of the flow rate, and lack of ability to control the frequency of pulses.
Analysis of existing methods and designs to create air bubbles and liquid flow pulsating regimes shows the need for a new design that would be simple to implement, does not require the use of electric drives for rotating parts of the device or a source with higher liquid pressure. The design of a new device must be capable of producing bubbles with diameters in a wide range of sizes from micrometers to millimeter and must not include elastic deformable elements that would impose restrictions on the time action, liquid flow values, and ability to control the frequency and amplitude of the pulsations. In addition the new device must require low energy consumption.
It is a purpose of the present invention to provide a generator for the creation of air bubbles and liquid flow pulsating regimes that is capable of producing bubbles with diameters in a wide range of sizes, does not include elastic deformable elements, and has low energy consumption.
Further purposes and advantages of this invention will appear as the description proceeds.